www.sefindia.org

[Rudra Nevatia]

It is proposed to change Clause 7.9.2 of IS:1893 which presently reads as
follows:

The design eccentricity, edi to be used at floor i shall be taken as:

edi = 1.5esi + 0.05bi  or
esi - 0.05bi

whichever of these gives the more severe effect in the shear of any frame

where:

esi = Static eccentricity at floor i defined as the distance between centre of
mass and centre of rigidity, and

bi  = Floor plan dimension of floor i, perpendicular to the direction of force


NOTE -  The factor 1.5 represents dynamic amplification factor, while the
factor 0.05 represents the extent of accidental eccentricity


The proposed change envisages changing factor 1.5 to 1.0 and factor 0.05 to
0.1.

My understanding of the clause in its existing form is as follows. Please
correct me if I am wrong.

In its general form the expresion for edi should read:

edi = A*esi + B*bi

Factor A accounts for possible coupling of torsional and lateral modes of
vibration and depends on the ratio of frequencies in the two modes. As this
ratio approaches 1, resonance will set in. Hence a multiplier of 1.5 on esi.
FEMA relates factor A to the extent of torsion irregularity and in extreme
case, this factor can reach up to 3!

Factor B accounts for possible variation of ground motion along the width
of building. Obviously, the factor can take a positive or negative value.

Two extreme cases are possible:

1. Torsional coupling occurs and accidental eccentricity is in the same
direction as the static eccentricty which is reflected by the first
equation of the present clause

2. Torsional coupling does not occur and the static eccentricty is in the
direction opposite to the static eccentricty which is what the second
part of the equation implies.

The proposed change would wrongly

- ignore the effect of coupling of lateral and torsional modes

- give benefit of reduction in torsional shear


May I therefore suggest that the clause be retained as it is.

Rudra Nevatia




          
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Posted via Email

[alpa_sheth at vsnl.com]

Dear All,
I too have concerns about the proposed revision to Clause 7.9.2 of IS:1893
but for different reasons:
I feel that the revision from 0.05 to 0.1 bi unnecessarily and perhaps
unconsciously punishes perfectly symmetrical structures.
ASCE 7-02 also has min torsional design mom based on actual torsion (Mt) +
that caused due to displacement of mass by distance of 5% of dimension of
structure perpendicular to direction of seismic force (Mta). In higher
seismic zones, Mta is to be multiplied by a factor of        ((max
displacement/(1.2* avg displacement)) ^2.
Thus for a perfectly symmetrical structure,  where max displacement = avg
displacement, Mta would be based on eccentricity of 5%  *  (1/1.2)^2= 0.69
or very roughly 3.5% eccentricity.
On the other hand, the proposed revision to IS 1893 will increase the
accidental torsion from 5% bi to 10% bi for perfectly symmetrical structures

Regards,
Alpa
----- Original Message -----
From: <rudra_nevatia@yahoo.com>
To: <alpa_sheth@vsnl.com>
Sent: Monday, September 20, 2004 11:33 AM
Subject: [Econf] Clause 7.9.2 of IS:1893

[mc.upadhyay1]

õ



Hello Madam Alpa

you yourself (being administrator of SEFI) did not follow the following
dont's

* Please delete the message you are responding to from the body of your
email so as to limit the size of the email.

Regards

Mukesh Upadhyay

Posted via Email

[alpa_sheth at vsnl.com]

Yes,  I agree and plead guilty )))
But I think what we really mean is that if there is a loooong thread of
discussion on a topic we should not carry all of the thread in the reply so
as to limit email size. Retaining the most recent email on the topic in the
reply  is desirable so that  people  know the reference of the reply.
Cheers,
Alpa

Posted via Email